This invention relates to lubricating greases. More particularly, this invention is directed to lubricating greases useful at high operating temperatures.
In may applications lubricating greases are superior to lubricating oils due to the plastic nature of the greases. They offer the advantage of simplifying machine constructions because the sealing of the roller bearings, which is necessary when lubricating toils are employed, can be omitted. The greases themselves protect roller bearings from dirt and water by the formation of a cushion. Today, high-grade greases frequently have the same life as the roller bearings, which means that post-lubrication is unnecessary provided the maximum operating temperature of the grease is not exceeded.
Of the various types of greases, aluminium, calcium, sodium and lithium soap greases have become particularly known. Lithium soap greases have been widely used for many years. Their extensive use is due to the fact that they combine significant advantages of calcium and sodium soap greases, i.e., they have good resistance to water, and with dropping points of about 200.degree. C., may be used at operating temperatures of from 120.degree. C. to 150.degree. C.
However, due to a technical advance, it is often necessary today to control temperatures which, at least temporarily, are far in excess of the above-specified maximum temperature range for application of the conventional lithium soap greases.
For example, it has been found that for the lubrication of wheel bearings of some types of passenger cars the temperature stability of the known greases is no longer sufficient.
On mountain roads where frequent braking is required, bearing temperatures of 180.degree. C. to 200.degree. C. have been measured, especially in cars fitted with disc brakes mounted near the wheel bearings. When lithium soap greases of conventional type are used in such cases, there will be the risk that, under such strains, the greases will leak from the bearings and possibly smear the disc brakes.
A further example is the lubrication of hot-air fans. Here, too, temperatures of 150.degree. C. to 200.degree. C. may be reached.
In the course of technical development, lithium complex greases have been developed for some time which have superior temperature stability when compared to the conventional lithium soap grease which mostly consist of the lithium soap of 12-hydroxystearic acid as thickening agent.
However, none of these greases have proven to be satisfactory for all high operating temperatures and, in most cases, are not effective.
Thus, it is an object of the present invention to provide lubricating greases having operating temperatures in excess of the requirements of any operation in which they may be used.